Monocrystalline silicon is a basic building block for optical fiber devices such as optical fiber plugs, laser array modules and switch modules. Such optical fiber devices are typically fabricated by etching a wafer of monocrystalline silicon to establish one or more V-grooves, each V-groove being dimensioned to seat an individual optical fiber. The wafer is then sectioned to create a plurality of a submounts, each mounting a plurality of fibers in a set of V-grooves for alignment with those held by another such submount.
The V-grooves in a silicon submount are usually created by anisotropically etching the monocrystalline wafer whereby the wafer etches faster in certain crystallographic directions than in others. By selecting the appropriate wafer orientation and by selecting the mask pattern, features, such as V grooves, squares or pits, can be formed in a major surface of the wafer very precisely. In the case of V-grooves etched into a silicon submount, high precision is crucial. In some instances, the symmetry of the V-grooves (i.e., the orientation and lateral spacing of each V-groove from a reference point on the upper surface of the submount) must be better than 0.5 .mu.m.
In practice, silicon wafers of the type used in the microelectronics industry are diced from a single crystal ingot such that each wafer has its upper surface slightly tilted (typically about 3.degree.) from the wafer's vicinal or true (100) crystal plane along either the [011] or [011] direction after polishing. Typically, the wafer surface is titled with respect to the true (100) plane to facilitate wafer oxidation and other surface deposition steps performed during processing of the wafer, for example, to provide at least one masking layer over the monocrystalline silicon.
During processing, a reference flat is ground into each wafer parallel to the &lt;110&gt; direction. Although all wafer manufacturers grind a flat in each wafer, there is no standard regarding the orientation of the flat. For different manufacturers, the flat may be oriented differently. Thus, it is not possible to determine the orientation of the wafer, and more particularly, the orientation of the tilt axis, simply by reference to the location of the flat.
In order to produce V-groove features in a silicon wafer, the wafer is coated with a thin (e.g., .about.1 .mu.m thick) layer of a first masking material on both of its major surfaces. The first masking layers may be fabricated from silicon dioxide or silicon nitride for example. After the first masking layer has been applied, then a conventional photoresist mask is applied, thus forming a second masking layer over the silicon. Conventional photolithographic techniques are utilized to create a pattern of windows in the photoresist. The photoresist is etched along with the first masking layer to expose the silicon wafer through the pattern of windows. A first masking layer fabricated from silicon dioxide may be etched with a buffered hydrofluoric acid solution. Once the photoresist and first masking layer have been etched, the photoresist is stripped from the wafer, and then the wafer is anisotropically etched with a ethylene diamine procatechol solution to create the desired V-grooves through the windows in the first masking layer. Lastly, the first masking layer is stripped, and the wafer is sectioned into individual submounts. The silicon exposed through the windows in the first masking layer is typically anisotropically etched to create the desired features (i.e., the V-grooves) that lie in the wafer vicinal (100) crystal plane. When V-grooves are etched parallel to the &lt;110&gt; direction, the planar facets bounding the V-grooves lie within {111} planes that are inclined with respect to the (100) plane at an angle of 54.7.degree..
Often, the desired precision of the etched V-grooves is not obtained, requiring that the wafers be screened, adding to production costs. Thus, there is a need for a technique that improves the positional accuracy of features etched in a crystalline body.